This investigation has been designed to define the pathomechanism of the bullous diseases, pemphigus and pemphigoid. The subclasses of IgG antibodies have been determined. It has also been found that total complement and complement components are decreased within blister fluid. Attempts to reproduce the disease in animals has not been successful, but lymphocytes from two patients have been found to release macrophage aggregation factor in the presence of a soluble epidermal extract. The role of cellular immunity in the pathomechanism of these diseases will be further explored.